I may be off I haven't taken math classes in years, but it seems to me that "maps onto" in this case is another way to say intersects with. As in at what x value do the 2 functions have the same y value. So you set them equal to each other and solve for x.

"Maps onto" is more useful if talking about 3 dimensions where a 3d object intersects a plane, and leaves what we see as a 2d map when we look at all the points of intersection. Think of a sphere intersection a plane. We would see a map of a circle.